Fuel fabrication processes
Uranium dioxide is the most extensively used and commercialized nuclear fuel for thermal and fast reactors.
Uranium dioxide advantages:
·high melting temperature (2780°С);
·chemical resistance to the main coolant types (light and heavy water, sodium, carbon dioxide);
·good compatibility with cladding materials (stainless steel, zirconium alloys) at operating temperatures;
·possibility of producing high-density pellets;
·acceptable radiation resistance at neutron fluxes of ~1014 n/cm2×s and fluences of ~1022 n/cm2, i.e. for 3 years;
·isotropic crystal lattice, which facilitates high-temperature sintering.
Uranium dioxide disadvantages:
·low heat conduction dropping abruptly as temperature increases. This accounts for sharp temperature differentials between pellet center and
periphery (DТ~1000–1500 оС);
·easy oxidation in air. It needs inert dry environment or vacuum, otherwise the pellet will get saturated with moisture and oxygen will be adsorbed by its surface layer. Moisture on the surface can cause hydrogenation of the cladding and failure of the fuel element;
·presence of oxygen moderates the neutron spectrum in a fast reactor and reduces the breeding ratio.
Steps in production of uranium dioxide pellets
1. Conversion of uranium hexafluoride into dioxide: a) “Wet” AUC process:
uranium hexafluoride is passed through aqueous solution of ammonium carbonate (NH4)2CO3, producing a solid insoluble precipitate of ammonium–uranyl–carbonate (AUC) – (NH 4)4UO2(CO3)3;
heat treatment of AUC at 550–650 оС, resulting in its thermal decomposition with formation of UO2 as fine powder;
b) “Dry” process:
uranium hexafluoride hydrolysis by steam at 150–300 оС to produce uranyl fluoride UO2F2:
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UF6 + 2H2O → UO2F2 + 4HF;
UO2F2 pyrohydrolysis by steam and hydrogen at t 550 оC to form fine UO2 powder and hydrofluoric acid (HF):
UO2F2 + H2 → UO2 + 2HF.
The resulting fine powder of UO2 is made unfit for compaction by its very small particles (less than 0.6 μm). To obtain larger particles, the following operations are carried out:
2.Mixing with plasticizing agents;
3.Hydrocompaction: filling of a rubber mould; placement in a container with liquid, which is then pressurized (uniform compression), briquetting;
4.Granulation by briquette milling;
5.Annealing to remove plasticizers;
6.Cold molding to make pellets;
7.Pellet sintering;
8.Pellet quality control and sorting according to size, carbon content (plasticizers), and stoichiometry.
As regards the technology of producing mixed oxide (MOX) fuel, there
are three possible blends:
PuO2 + 238UO2, where Pu is taken from weapons materials;
PuO2 + 238UO2, where Pu is obtained by irradiated fuel reprocessing;
and
235UO2 + 238UO2, where 235U is taken from weapons materials.
MOX fuel production presupposes availability of two source materials: 238UO2 powder made from depleted or natural uranium; and
PuO2 coming from weapons or reactors, or 235UO2 from weapons uranium.
It is impossible to guarantee homogeneity of the (Pu, 238U) or (235U, 238U) mixture. Mixing of powder from different sources is the only stage to distinguish the production method of MOX fuel from that of uranium dioxide fuel. Mixture homogeneity is essential to reactor safety. As power rises, fissile isotopes will be the first to heat up. Doppler widening of capture and fission resonances takes place with the ensuing cumulative positive effect of reactivity. A fertile isotope takes more time to heat up, bringing about a negative Doppler effect of reactivity. This delay is the shorter, the higher is the mixture homogeneity. With poor mixing, the
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positive effect of reactivity of fissile isotopes can cause power rise to an unacceptably high level, before the negative effect of reactivity of the fertile isotope can have its stabilizing action.
Fuel fabrication stages
1.Preparation of nuclear fuel (conversion of UF6 into UO2; powder production; pelletization and sintering);
2.Preparation of tubular claddings and end-pieces;
3.Preparation of fuel assembly components.
4.Fitting up of fuel rods: packing of tubular claddings with pellets; attachment of end-pieces; filling with helium; sealing of end-pieces; fuel rod quality control.
5.Putting together of fuel rods into assemblies; quality control; rig tests. Fabrication of fuel rods and assemblies is:
∙a precision process;
∙an automated quantity production process;
∙an important object of physical protection, accounting and control of nuclear materials.
It takes hundreds of thousands of components and millions of fuel pellets to build a reactor core.
Fuel use in nuclear reactors
A reactor is above-critical before its operation starts, but the reactivity margin is suppressed by special controls, such as absorber rods, boric acid in the coolant, burnable poison in fuel. Fuel burning and buildup of fission products (FP) cause the reactor to go subcritical (Кeff < 1). For the reactor to continue operating, Кeff should be raised above unity. The primary purpose of refueling is to restore the reactivity margin.
Another purpose is to flatten power density distribution for the highest possible energy generation and uniform fuel burnup.
These purposes may be attained by:
∙full or partial refueling;
∙rearrangement of fuel assemblies with different burnup in the core;
∙combining of the first two approaches.
Refueling of nuclear reactors can take the following forms:
Cyclic refueling, involving uniform fuel distribution and its complete replacement once the reactivity margin is exhausted.
The disadvantages of this approach include:
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∙non-uniform power density distribution in the reactor;
∙quick burnup of the central core, with the reactivity potential of the peripheral fuel retained.
Partial cyclic refueling
It is only the fuel assemblies that have reached their burnup limit that will be unloaded from the reactor and replaced with fresh ones. The core is divided into a number of concentric zones. During each subsequent refueling, the burnt assemblies will be replaced with fresh ones in turns, one zone after another, from the center to the periphery.
The advantage of this approach lies in the same burnup of unloaded fuel.
A disadvantage of this approach is that fresh fuel will take time in moving towards the periphery. This results in impaired power density distribution, with its higher values found in the central zones.
Scattered refueling
The core is divided into groups of fuel assemblies, each including the same number of FAs, e.g., four. The assemblies replaced in the first refueling operation will all be itemized under No. 1. During the second refueling, this procedure will be repeated for assemblies No. 1 and 2, and so on. The distribution of fresh FAs will be uniform throughout the core, resulting in improved global uniformity of power density distribution.
Refueling “from the periphery to the center”
The core is divided into concentric zones containing the same number of fuel assemblies. The first unloading operation will involve the FAs of the central subzone with the highest burnup. They will be replaced by FAs of the second subzone, the place of the latter will be taken by assemblies of the third subzone, etc. The last subzone vacated will be filled with fresh fuel assemblies.
As a result, FAs concentrating in the center will have the highest burnup, i.e. they will be less reactive than the peripheral assemblies. The associated processes are depression of heat release in the center and reduction of the efficiency of reactor controls.
Modified scattered refueling
This method involves the following:
1.The peripheral ring of fuel assemblies is singled out, comprising, e.g., 1/5 of all the reactor’s FAs;
2.The central core area is divided into local groups, four FAs in each;
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3. Assemblies No. 1 are removed from each group during the first refueling and are replaced by FAs from the outer ring. The vacated outer row is loaded with fresh FAs.
The advantage of this method lies in uniform power density distribution, without a surge in the center peculiar to “partial cyclic” refueling, or depression in the center characteristic of the “periphery-to center” process.
Refueling procedures
Refueling may take place:
∙after reactor shutdown and cooldown, with its vessel head removed;
∙after shutdown, without cooldown and vessel head removal;
∙at low or full power.
A full shutdown procedure is practiced at light water reactors. Once a
year, a reactor will be shut down for 4 to 6 weeks, its vessel head removed, the irradiated fuel assemblies unloaded, the remaining FAs reshuffled, and fresh fuel loaded. All these operations proceed under water.
In a fast reactor with liquid metal coolant, fuel is reloaded after reactor shutdown, with its vessel head left in place. Use is made of a rotary plug with a reloading mechanism. Two eccentric systems guide the mechanism to an appropriate FA so that it can grip the assembly by its top end-piece and move it into the in-pile storage at the core periphery. Fuel assemblies are kept for some time there to be later removed by a simpler mechanism during reactor operation.
Heavy water reactors, such as CANDU, can be refueled without shutting down the reactor or reducing its power. Fuel assemblies are placed in horizontal channels. The refueling process relies on the “sluice” principle. There is a reloading machine on either side of the reactor. Each machine has a sleeve, which is connected to either end of the fuel channel. Once the channel plug is removed, pressure in the machines and in the channel levels off, fuel is unloaded, and the channel is closed again. One of the machines inserts a fresh assembly at one end, while the other picks up the assembly pushed out at the opposite end.
RBMK reactors also feature on-load refueling. To this end, a reloading machine is used and the “sluice” principle is appli ed, as is the case with CANDU:
∙the reloading machine, filled with condensate, is joined to a channel;
∙pressure in the machine box becomes equal to that of the channel;
∙the depressurized channel receives cold condensate;
∙the spent FA is gripped and retrieved;
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